One form of process for producing hollow bodies of thermoplastic material, as can be found for example in German specification DE-AS No 1 178 580, provides that a tubular preform of plasticised material is expanded between parts of a blow molding mold enclosing the preform, by means of an increased internal pressure within the preform. Prior to closure of the mold halves to constitute the closed mold cavity, the preform is spread transversely with respect to the extrusion direction and in a separation plane of the blow molding mold, so that the preform extends beyond the lateral edges of the mold cavity. With that process, the preform is drawn apart by means of spreading mandrels, immediately prior to closure of the blow molding mold, in such a way that the preform extends substantially over the entire width of the mold cavity before the mold halves are closed. That process is used primarily for producing hollow bodies of a flat cross-section in which the width of the hollow body is a multiple of its depth.
If the production of such articles involves extruding a usual tubular preform of circular cross-section, then expansion of the preform in the blow molding mold entails extremely detrimental distribution of the material in the wall of the finished product as the tube constituting the preform has to be very greatly stretched in one direction during the expansion procedure whereas it undergoes scarcely any stretch at all in a direction perpendicular thereto.
In order to alleviate irregularities which are produced in that fashion in terms of the wall thickness of the finished product, above-quoted DE-AS No 1 178 580 involves spreading the tube constituting the preform prior to closure of the mold halves, in the separation plane of the blow molding mold. That provides that the initial configuration of the tubular intermediate article, which in itself is unsuitable for flat products, is suitably modified prior to the blow molding mold being closed. In this process the tubular preform is initially extruded over spreading mandrels which extend approximately over the entire height of the blow molding mold. Moving the spreading mandrels away from each other transversely with respect to the extrusion direction in the separation plane of the blow molding mold causes the preform to be spread out flat in the desired configuration. The mold halves of the mold are then closed. Prior to removal of the finished article from the mold the spreading mandrels have to be removed again therefrom. For that purpose the spreading mandrels are withdrawn from the bulge portion formed in the molded component outside the mold, in the extrusion direction.
It will be appreciated that this is difficult, particularly when producing large components of a preform length of one meter and more, as an appropriate amount of space has to be provided for the stroke movement to be performed by the spreading mandrels in the extrusion direction, while on the other hand there is a risk that the operation of withdrawing the mandrels after the blow molding mold is closed may take so much time that the material entirely or partially hardens around the spreading mandrels, and that would cause considerable difficulty in withdrawing the mandrels. For that reason the above-outlined process of DE-AS No 1 178 580 has not been put to use in a true practical context.
As an alternative to that procedure the preform can be expanded prior to closure of the blow molding mold, by a pre-blowing procedure.
It will be noted however that, in the production of large blow moldings, that procedure gives rise to problems insofar as a comparatively long period of time is required until the air has escaped from the preform, before the blow molding mold is closed. The larger the blow molding, the correspondingly longer is the cycle time that this entails, and that is not desirable for reasons of cost and in part is not truly practical from the technical point of view.